As far as shorties and belly buttons go, lol, there isn't always a correlation...
I'm 4'10" and I went from 108 lbs. up to 188lbs. with my first babe (yes very bad GD). I never had an outie belly button. I never got stretch marks on my belly ( I did get them on my hips and the back of my legs near the bottom of my butt). But my belly button...never popped. I thought that was a gimme with pregnancy and was kinda lookin forward to it to be honest. Now I'm pregnant with baby number two and still no poppage lol...
And the 'line' that you are talking about is in fact the linea negra. Not everyone gets it. I've also heard of some women who are darker pigmented getting a lighter line up the middle...so who knows...its directly linked to the same thing most everything else is...hormones.
Happy prego,
Sara